Nonabilities: Some creatures lack certain ability scores. These creatures do not have an ability score of 0—they lack the ability altogether. The modifier for a nonability is +0. Other effects of nonabilities are detailed below.
Constitution: Any living creature has at least 1 point of Constitution. A creature with no Constitution has no body or no metabolism. It is immune to any effect that requires a Fortitude save unless the effect works on objects or is harmless. The creature is also immune to ability damage, ability drain, and energy drain, and automatically fails Constitution checks. A creature with no Constitution cannot tire and thus can run indefinitely without tiring (unless the creature’s description says it cannot run).
So undead don't need to save against Stinking Cloud; it doesn't work on objects and it isn't harmless. They do need to save against Disintegrate, for example, which has a Fort save but works on objects. They don't get to use their Cha modifier; they use their Con modifier, which is +0 since it is a nonability.
-Hyp.
